Log-periodic antenna arrays



22,1970 R. .,1. F. GUERTLER ETL 3,550,138 E Los-PERIODIC ANTENNA ARRAYs Filed July 5, 1968 1 4 Sheets-Sheet 5 ALL BAYEOUAL POWER0NE BAY FED 780 OUT UP PHASE 400 MHZ. PHASE CENTR TU ARRAY CENTRE 0.28 l

- lnuenlar RUOLF J. F. GUER TLR RA YMoNo J. c. @A wsa/v llame y DeC- 22, 1970 R.J. F. GUERTLER ETAL 3,550,138

I LOG'v-PERIODIC ANTENNA ARRAYS Filed July s, 195e i sheets-sheet L l l l l l l l I I fi/ QZ STEER/NG CONTROL LEVER v nvenlors RUOOLF J. F, QUERTLER RAYMOND J, cf RAWSON United States Patent O 3,550,138 LOG-PERIODIC ANTENNA ARRAYS Rudolf Julius Franz Guertler, Eastwood, New South Wales. and Raymond John Clancy Rawson, Lane Cove, New South Wales, Australia, assignors to International Standard Electric Corporation, New York, N.Y., a corporation of Delaware Filed July 3, 1968, Ser. No. 742,303 Claims priority, application Australia, July 21, 1967, 24,889/ 67 Int. Cl. H01q .7l/10, 3/26 U.S. Cl. 343-751 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF T-HE INVENTION Field of the invention The invention relates generally to a log-periodic antenna array which permits in the horizontal plane either omnidirectional radiation or unidirectional radiation electronically steerable in any asmuthal direction, over a wide frequency band.

Description of the prior art An array of suitably placed vertical dipoles could provide, with suitably selected feed point voltages, either an omnidirectional or an unidirectional radiation pattern. However, the patterns and the input impedances will change considerably when the operating frequency is changed over a wide frequency band. The steering equipment required would be quite complicated due particularly to the change of the input impedances with frequency.

SUMMARY oF THE INVENTION The object of the invention is to provide a fixed antenna or antenna array which permits in the horizontal plane either omnidirectional radiation or unidirectional radiation electronically steerable in any asmuthal direction, over a wide frequency band.

According to the invention, an antenna array comprising at least three log-periodic antennas mounted with coincident apices and equal angles between adjacent antennas is driven, by voltages supplied to each antenna, whose phase and amplitude are controlled to obtain an omnidirectional radiation pattern or a unidirectional radiation pattern and steering of the unidirectional radiation beam.

The above mentioned and other objects of this invention will become apparent by reference to the following description with the accompanying drawings in which:

FIG. l is a schematic plan view illustrating an antenna array comprising three vertically poralized log-periodic dipole antennas.

FIG. 2 is a schematic plan view illustrating an antenna array comprising two vertically polarized log-periodic dipole antennas.

FIG. 3 is a schematic plan View illustrating an antenna 3,550,138 Patented Dec. 22, 1970 array comprising three vertically polarized log-periodic dipole antennas.

FIG. 4 is a schematic diagram illustrating the connection of an inductor to a dipole of an antenna.

FIG. 5 illustrates a radiation pattern achieved when all three antennas are driven with voltages of equal amplitudes and equal phases.

FIG. 6 illustrates an unidirectional pattern achieved |when all three antennas are driven with voltages of equal amplitudes one of which is 180 out of phase with the other two.

FIG. 7 is a schematic diagram illustrating a circuit which is used to control the amplitude and phase of the signal which is delivered to each of the separate antennas.

The requirement to operate an antenna over a wide frequency range suggests use of log-periodic antennas. FIGS. l and 2 show schematically the plan view of arrays consisting of vertically polarized log-periodic dipole antennas FlGl, F2G2, F3G3, F designates the position of the shortest dipole and feed point. G designates the position of the longest dipole in each log-periodic element. A, designates the virtual apex of a log-periodic element. P designates the position of the phase center at a specified frequency. The distance phase center-apex, i.e., AlPl, A2P2, etc., remains substantially constant in terms of wave length. If a single element is driven in the array illustrated in FIG. l, maximum radiation is obtained in direction Q1; if the log-periodic element is designed with at least beam width in the H plane, this mode of operation may be used for communication over Q1i60 asmuths. For asmuths outside the range Q1i60", one of the other log-periodic elements, F2G2 or F3G3, may be driven. This type of operation does not satisfy certain specifications of steerability -which may require feeding of two or three of the log-periodic elements with voltages of suitable amplitude and phases. This operation is quite frequently dependent. The phase centers, P1, P2, P3, move with increasing frequency towards the apices A1, A2, A3, that is, the distance of the phase centers from the array center 0, increase with increasing frequency, the radiation patterns change with frequency, and the coupling effects between active dipoles of adjacent log-periodic elements change with frequency. If all driving voltages have equal amplitude and equal phases, an omnidirectional pattern would result; it will be, however, frequency dependent and Awill show increase lobe-splitting with increasing frequency.

The array illustrated in FIG. 2, uses two log-periodic dipole elements arranged with a common apex A. It gives maximum radiation in direction Q0 when both elements are fed with voltages of equal amplitude and equal phase. This array is used for steering of the maximum radiation direction, so it provides steering within a limited range of asmuthal direction QoiAQ only.

FIG. 3 shows schematically the plan view of an array according to the principle of the invention. It consists of three log-periodic dipole elements, F`1G1, F2G2, F3G3, spaced equi-angularly with apices coincident with 0, the center of the array. Each log-periodic element, if isolated, would radiate in direction F0, that is towards the center of the array. Omnidirectional operation is obtained when the driving voltages, applied at F1, F2, F3, are of equal amplitudes and equal phases. Unidirectional operation and steering is obtained, by suitable adjustment of amplitude and phase of the individual driving voltages.

The distance of the phasecenters P1, P2, P3, from the array center 0 is constant in terms of wavelength. This secures a practically frequency independent operation, that is, for any set of relative voltage amplitudes and voltage phases the radiation pattern and the individual feed point impedances will be essentially independent of frequency,

corresponding to the frequency response of the log-periodic elements.

The shape of the omnidirectional radiation pattern and of the various unidirectional radiation patterns is determined by three design parameters, the number of elements, the beam width in the horizontal plane and the distance between the phase centers P1, P2, etc., and the array center zero. For instance, omnidirectional operation, that is, driving voltages of equal phase and equal amplitude, will generally produce a pattern with regularly displaced maxima and minima. A reduction of the ratio of maximum to minimum eld strength is obtained by increasing the number of log-periodic dipole elements, for instance, from three to four to five or six, and/or by increasing the beam width and/or reducing the phase center-apex distance of the log-periodic element used in the array. The latter two changes require a change of the design. Beam width and phase center-apex distance are interdependent to a certain degree, but the changes have similar effects.

Some modes of operation require that one element, for instance FlGl in FIG. 3, be non-radiated. This is achieved by supplying a voltage of suitable ampliude and phase to F1 which produces currents in the dipoles of the active region equal and out of phase to currents produced by the eld from the other elements. At higher operating frequencies it can happen that long dipole halves, at and near G1 may resonate. This resonance can lbe surpressed in accordance with the invention by inserting an inductance X1, X2, as illustrated in FIG. 4, approximately at the center of long dipole halves. These inductances suppress possible higher frequency currents in long dipole halves, and at low frequencies they provide additional loading of the dipoles permitting a reduction of physical length. The inductors may be coils with or without ferrite cores or may be ferrite rings surrounding the dipole conductors.

An array incorporating the invention consisting of three log-periodic elements as illustrated in FIG. 3 was measured under various conditions. FIG. 5 shows the radiation pattern when all three elements were driven with voltages of equal amplitude and equal phase. When the phase of the feed point voltage of one element was reversed, the pattern illustrated in FIG. 6, was obtained. It is in principle an unidirectional pattern, and it can be improved with respect to the other two elements.

While the specific description has been of antenna array in which the elements are vertically polarized, the invention can also be employed with arrays employing other directions of polarization. l

A joy-stick control lever may be manipulated to adjust the power level and phasing of all elements simultaneously, enabling the operator to select any one of the required propagating modes.

For omnidirectional propagation the control level is positioned centrally; and control arrangements feed all elements with voltages of equal phase and amplitude.

When the control lever is moved, the amplitude and/or phase of the various element feeds are controlled so that maximum possible radiation in a selected asmuthal direction is obtained.

A schematic diagram is illustrated in FIG. 7 showing one way in which pairs of inductors and differential capacitors in each of the feeds, to three antennas, in aecordance with this invention can be varied by the joystick lever.

It is to be understood that the foregoing description of specific examples of this invention are made by way of example only and are not to be considered as a limitation on its scope.

We claim:

1. An antenna array comprising at least three log periodic antennas mounted with coincident apices and equal angles between adjacent antennas to provide a wide frequency range of operation,

means for applying a voltage to each antenna, and

means for controlling the amplitude and phase of said applied voltage to obtain a predetermined radiation pattern,

said control means comprising a voltage source, a plurality of series tuned circuits each coupled to one of said antennas and connected across said voltage source and a steering control lever mechanically coupled to each of the tuned circuits for varying the tuning thereof.

2. An antenna array according to claim 1 in which said antennas are dipole antennas.

3. An antenna array according to claim 2, in which said antennas each include an inductor at the center of each of the longer dipole halves to suppress resonance.

4. An antenna array according to claim 3 wherein said inductor is a ferrite core coil.

5. An antenna array according to claim 3 wherein said inductor comprises a plurality of ferrite rings.

Log-Periodic Antenna Structures, by Isbell Research Studies on Problems Relating to ECM Antenna-AF 33(610)3200, Report #9 Published at Wright Field, Feb. 20, 1958, 1 pp. of dwg., 2 shts. of spec. 343-908.

ELI LIEBERMAN, Primary Examiner S. CHATMON, JR., Assistant Examiner U.S. Cl. X.R. 343-854, 792.5 

